1. Field of the Invention
The present invention relates to a digital radio receiver for a portable telephone, a car phone, etc. which is provided with a digital signal processor (DSP). More particularly, the present invention relates to a digital radio receiver equipped with an algorithm designed to utilize the DSP, which functions to demodulate a digital signal, in order to minimize the need for manual adjustment, adjusting circuits, and expensive components, ensure freedom from the dependence on temperature changes, achieve equal inphase component (hereinafter referred to simply as "I") and quadrature component (hereinafter referred to simply as "Q") amplitudes, eliminate DC offset, and permit proper demodulation even if I and Q are not orthogonal.
2, Description of the Related Art
A conventional digital radio receiver will be described with reference to FIG. 27, FIG. 28, and FIG. 29. FIG. 27 is a block diagram showing the configuration of a demodulating system of the conventional digital radio receiver; FIG. 28 shows the DC offset eliminating circuit illustrated in FIG. 27; and FIG. 29 shows the IQ amplitude imbalance correcting circuit illustrated in FIG. 27.
In FIG. 27, the conventional digital radio receiver is equipped with an antenna 1, an RF/IF converter 2 for converting radio frequencies to intermediate frequencies, an IQ separator 3 for separating I and Q and for issuing differential outputs Ip, Qp, In, and Qn (the suffix of "p" denotes positive; the suffix tn "n" denotes negative), a filter 4 for eliminating a noise such as a radio frequency, a DC offset eliminating circuit 5 for removing a DC offset from an output of the filter 4, an IQ amplitude imbalance correcting circuit 6 for correcting the IQ amplitude imbalance of an output of the DC offset eliminating circuit 5, a multiplying sampler 7 for sampling an output of the IQ amplitude imbalance correcting circuit 6, and a digital signal processing unit 8 comprised mainly of DSP for performing quadrature phase demodulation.
In FIG. 28, the DC offset eliminating circuit 5 is equipped with resistors 9 through 11 and 13, a differential amplifier 12, resistors 14 through 16 and 18, and a differential amplifier 17. It should be noted that two offset eliminating circuits are required because there are two signal lines for I and Q.
In FIG. 29, the IQ amplitude imbalance correcting circuit 6 is equipped with resistors 19 and 20, and variable resistors 21 and 22.
In the elimination of the DC offset by the DC offset eliminating circuit 5, the occurrence of DC can be prevented by subjecting I and Q to differential amplification as illustrated in FIG. 28.
The IQ amplitude imbalance correcting circuit 6 requires adjustment at the time of manufacture if IQ amplitude imbalance is found. The imbalance in the IQ amplitude can be corrected by adjusting the variable resistors 21 and 22 shown in FIG. 29.
In the manufacture of the digital radio receiver which employs the quadrature phase modulator-demodulator, it is difficult to obtain equal I and Q amplitudes and to eliminate the DC offset because of the variations in components. For this reason, manual adjustment is required at the time of manufacture to cope with the IQ amplitude imbalance. In addition, expensive components and large-scale circuits are required to control the variations in the components.
Further, expensive components which exhibit less temperature-dependent change in characteristics are often used or the circuit scale of a temperature compensation circuit tends to grow larger to deal with the temperature changes encountered during actual use. This presents difficulties in achieving compact design and lower cost. In other words, expensive components and large-scale circuits are necessitated to receive intact input signals without depending on temperature changes.
The conventional digital radio receiver described above poses a problem in that the DC offset eliminating circuit 5 and the IQ amplitude imbalance correcting circuit 6 as well as the manual adjustment are required since it is difficult to obtain equal IQ amplitudes and remove the DC offset in the manufacture due to the variations in the components involved.
There is another problem in that the need for the expensive components exhibiting less temperature-dependent changes in characteristics or the temperature compensation circuit to cope with the temperature changes in actual use inevitably leads to higher cost and an increased scale of the circuit.